1. Field of the Invention
The present invention relates to a zoom lens system, and in particular, relates to a telephoto zoom lens system which has a zoom ratio of about 2.5 to 3.0, the angle of view of less than 10xc2x0 at the long focal length extremity, and is mainly used for a single lens reflex (SLR) camera.
2. Description of the Related Art
Telephoto zoom lens systems with a zoom ratio of about 3.0 have been known from, for example, the U.S. Pat. No. 5,347,399, and U.S. Pat. No. 5,530,594. In the U.S. Pat. No. 5,347,399, the telephoto zoom lens system includes a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object, and the first lens group is stationary while the second and third lens groups are moved upon zooming. In such a zoom lens system, the entire length thereof is advantageously maintained constant upon zooming; however, the entire length of the lens system cannot be made short, which is unsuitable for miniaturization.
Furthermore, the telephoto zoom lens system taught in the U.S. Pat. No. 5,530,594 likewise has a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object; however, each lens group is moved independently of each other upon zooming. In such a zoom lens system, since zooming is performed by moving all of the lens groups independently, freedom in the optical design thereof is increased, so that the correcting of aberrations, and miniaturization of the lens system can be advantageously attained. On the other hand, the embodiments of the U.S. Pat. No. 5,530,594 have disclosed thirteen lens elements, which is relatively large, so that difficulties arise in reducing the cost of the lens system. Furthermore, the traveling distance of the second lens group is relatively long, which has made the structure of the lens frame complicated.
It is an object of the present invention to provide a miniaturized telephoto zoom lens system which has a zoom ratio of about 2.5 to 3.0, the angle of view of less than 10xc2x0 at the long focal length extremity, a simple structure, and is produced at a low cost.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. At least the first lens group and the third lens group are moved along the optical axis to perform zooming. The second lens group includes at least one negative lens element, and the zoom lens system satisfies the following conditions:
0.65 less than fw/f1 less than 0.80xe2x80x83xe2x80x83(1)
1.73 less than N2n less than 1.85xe2x80x83xe2x80x83(2)
0.15 less than X3/fw less than 0.21xe2x80x83xe2x80x83(3)
xe2x88x920.05 less than X2/X1 less than 0.10xe2x80x83xe2x80x83(4)
wherein
fw designates the focal length of the entire zoom lens system at the short focal length extremity;
f1 designates the focal length for the first lens group;
N2n designates the average refractive index, with respect to the d-line, of the negative lens elements in the second lens group;
X3 designates the traveling distance, upon zooming, of the third lens group from the short focal length extremity towards the long focal length extremity, under the condition that the direction from the image plane towards the object is defined as the positive direction;
X2 designates the traveling distance, upon zooming, of the second lens group from the short focal length extremity towards the long focal length extremity, under the condition that the direction from the image plane towards the object is defined as the positive direction;
X1 designates the traveling distance, upon zooming, of the first lens group from the short focal length extremity to the long focal length extremity, under the condition that the direction from the image plane towards the object is defined as the positive direction.
It is preferable that the second lens group be stationary upon zooming. According to this arrangement, only the first and third lens groups are the moveable lens groups, so that a frame structure for supporting the lens groups can be simplified.
The first lens group is preferably constituted by a three-lens-element arrangement in which a positive single lens element, a negative single lens element and a positive single lens element are provided, in this order from the object; and the first lens group satisfies the following condition:
62 less than xcexd1pxe2x80x83xe2x80x83(5)
wherein
xcexd1p designates the average Abbe number of the positive lens elements in the first lens group.
Furthermore, the most image-side lens element in the third lens group includes a negative meniscus lens element having the concave surface facing towards the object, and the third lens group preferably satisfies the following condition:
xe2x88x920.90 less than f3/f3end less than xe2x88x920.65xe2x80x83xe2x80x83(6)
wherein
f3 designates the focal length of the third lens group; and
f3end designates the focal length of the most image-side lens element in the third lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-371586 (filed on Dec. 27, 1999) which is expressly incorporated herein by reference in its entirety.